My bad, I was looking for subject effects in paired t-tests. And I can now see SPM5 and SPM8 are equivalent. Thanks Donald.
Including the global mean intensity as a covariate has me intrigued. In principle, wouldn't it make sense to include these values as a nuisance effect in fMRI, just as in PET? I guess in the paired t-test case, any subject differences in global mean intensity should be removed by the comparison, but for a one sample t-test or a two sample t-test it would seem to make sense…(?)
Ta,
Rich
On 28/04/2012, at 12:24 AM, MCLAREN, Donald wrote:
> (1) If you include a column for each subject in a one-sample t-test,
> then the group effect will always have a t-statistic of Inf. This
> results from the residual being 0. As far as I know, this was never
> done in SPM5.
>
> (2) I do not have a strong opinion on this, except to say that your
> contrast will no longer be comparing TaskA v. TaskB or TaskA v.
> baseline. Rather, you will be comparing the globally-adjusted
> contrast.
>
> (3) What subject-specific are you referring to?
>
> From the wording in your email, it seems like you might be using
> multiple measurements per subject. In this case, you should not be
> using a one-sample t-test.
>
> Best Regards, Donald McLaren
> =================
> D.G. McLaren, Ph.D.
> Postdoctoral Research Fellow, GRECC, Bedford VA
> Research Fellow, Department of Neurology, Massachusetts General Hospital and
> Harvard Medical School
> Website: http://www.martinos.org/~mclaren
> Office: (773) 406-2464
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> On Fri, Apr 27, 2012 at 6:43 AM, Richard Morris
> <[log in to unmask]> wrote:
>> Dear list,
>>
>> I've noticed a change in the one-sample t-test design matrices produced by SPM8 compared to SPM5. I seem to remember SPM5 used to (automagically?) include subject effects, with columns for each subject in the design matrix, but SPM8 doesn't do that anymore. Is there a setting change I can make to reintroduce subject effects in my design matrix?
>>
>> On a related note, is there any reason not to include the global covariate (ANCOVA) in a one-sample t-test of fMRI data? I've noticed I often get more regionally specific activation with higher t-values in my results when I include the global covariate. I'm guessing this is because of subject-specific effects which I would like to remove/minimise.
>>
>> Best,
>>
>> Rich
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